Derivation of dilution factors for first-tier exposure assessment in Asian rivers using a high spatiotemporal resolution chemical exposure analysis model

In recent years, the environmental impacts of down-the-drain chemicals in consumer products have received increasing attention. For a comprehensive environmental risk assessment, dilution factors (DFs), which are used for first-tier exposure assessment, are lacking, especially for the direct discharge scenario. In this study, DFs were derived for 101 rivers in nine Asian countries considering two exposure scenarios for chemicals discharged from households into rivers, wastewater treatment plants (WWTPs) and direct discharge. Dilution factors were derived separately for both scenarios, with a high spatial resolution of 1 × 1 km. River flow analysis was performed using the chemical exposure analysis model, Global-SHANEL Asia, which was originally developed to estimate chemical concentrations in Asian rivers based on the National Institute of Advanced Industrial Science and Technology's standardized hydrology-based assessment tool for chemical exposure load (AIST-SHANEL). It can also estimate anthropogenic discharge based on population density. The DF values were taken from the lower percentile or minimum of those analyzed under low-flow conditions, because screening assessments require the use of realistic worst-case values. The 5th percentile of DFs for the direct discharge and WWTP discharge scenarios were calculated as 64-5,100 and 1.3-4.2, respectively, and were used to assess linear alkylbenzene sulfonate (LAS) exposure. Although the DFs for the direct discharge scenario were larger than those for the WWTP discharge scenario, the predicted environmental concentrations of LAS in the direct discharge scenario were higher in some countries, making a risk assessment based solely on WWTP discharge insufficient in some cases. This study provides basic information for an initial risk assessment of chemicals in high-concentration local areas.